1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and, more particularly, to a semiconductor integrated circuit having a current output sensor and a means for processing an output signal from the current output sensor.
2. Related Background Art
An output signal from a current output sensor is conventionally processed by a circuit having an arrangement shown in FIG. 1. Referring to FIG. 1, the current output sensor is a photodiode.
Referring to FIG. 1, a current output from a photodiode 1 must have a dynamic range of about 80 dB or more from a dark state to a bright state. When this current signal is processed by, e.g., a 5-V single power source, the signal is first logarithmically compressed by an operational amplifier 23 and a diode 33. An output V.sub.O1 from the operational amplifier 23 is ##EQU1## where V.sub.c is the reference voltage, I.sub.S1 is the reverse saturation current of the photodiode, and I.sub.OP is the output current from the photodiode.
The output V.sub.O1 is changed into an output V.sub.O2 by a transistor 43 for compensating a dark current. The output V.sub.O2 is ##EQU2## where I.sub.B is the value of a constant current 53, and I.sub.S2 is the reverse saturation current of the transistor 43.
The output .sub.O2 is therefore ##EQU3## For I.sub.S1 =I.sub.S2 ##EQU4## Referring to FIG. 1, a buffer 63 changes the impedance. An operational amplifier 93 adjusts a signal to an input range for an AD (analog-to-digital) converter in cooperation with resistors 73 and 83. An output V.sub.O from the operational amplifier 93 is ##EQU5## where R.sub.1 is the value of the resistor 73, and R.sub.2 is the value of the resistor 83. This output V.sub.O is A/D-converted and then transmitted to a CPU or the like.
In the conventional circuit, however, a plurality of circuit blocks such as an operational amplifier, constant current source, reference power source, and AD converter are required to process an output signal from the current output sensor. This increases the number of elements and the power consumption, resulting in a large chip size and high cost.